SELECTIVE SEPARATION OF HYDROGEN FROM C1/C2 HYDROCARBONS AND CO2 THROUGH DENSE NATURAL ZEOLITE MEMBRANES

Weizhu An,1 Paul Swenson, 1 Lan Wu, 1 Terri Waller, 1 Anthony Ku,2 and Steven M. Kuznicki1,

1Department of Chemical and Materials Engineering, University of AlbertaEdmonton, AB, T6G 2V4, Canada

2GE Global Research, 1 Research Circle, Niskayuna, NY 12148, USA

ABSTRACT

Zeolite membranes have been studied for decades, but are not sufficiently robust

for widespread practical application. We examine an unusual natural clinoptilolite

material which has been compacted over time into crystalline blocks containing

essentially no macroporosity. When sectioned, this material behaves as a solid,

continuous molecular sieve membrane. Untreated membranes sliced from this dense

material were found to have as much as two times higher ideal selectivity for H2 over

CO2 and C1/C2 hydrocarbons than would be predicted by Knudsen diffusion. 1.2 mm-

thick membrane sections modified by simple hydrothermal treatments and applied to the

separation of hydrogen from CO2, CH4, C2H4, and C2H6 demonstrated H2 permeance as

high as 5.210-7 mol m-2 s-1 Pa-1 combined with ideal selectivities of 57 (H2/CO2), 22

(H2/CH4), 98 (H2/C2H4) and 78 (H2/C2H6) at 25 C; and 13 (H2/CO2), 6 (H2/CH4), 26

(H2/C2H4) and 12 (H2/C2H6) at 500 C. These modified natural zeolite membranes were

thermally and chemically stable, and their hydrogen permeance was reproducible after

multiple temperature cycles. These unique natural zeolite membranes have the potential

to be engineered for high-temperature, energy-efficient industrial separation and

purification applications including hydrogen separation, and to serve as a model for the

To whom all correspondence should be addressed (Email: steve.kuznicki@ualberta.ca; Telephone: 1.780.492.8819; Facsimile: 1.780.492.8958).

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development of robust synthetic zeolite membranes with superior separation

characteristics.

KEY WORDS: zeolite, membrane, high temperature, hydrogen separation, CO2, H2.

1. INTRODUCTION

Hydrogen is in high demand as a feedstock for petroleum refining and upgrading,

as a key reagent in metallurgical, food processing and chemical manufacturing reactions,

and as a clean, zero-emission fuel [1-3]. The petrochemical industry, in particular,

consumes large amounts of hydrogen in multiple upgrading and refining processes,

including hydrocracking, hydrodesulfurization and hydrodenitrogenation [1]. Hydrogen

is manufactured either from fossil fuel (oil, natural gas and coal) as a secondary energy

resource or from hydrogen-containing resources such as water. Currently, 41 MM tons

of H2 is produced annually worldwide, predominantly (80 %) through energy-intensive

steam reforming of natural gas [1]. A key component of any steam reforming process is

H2 separation and purification by adsorption-based separation units. It is estimated that a

20 % improvement in separation/purification efficiency could reduce worldwide energy

consumption for H2 production by 450 trillion Btu per annum [1]. An alternative to

steam reforming is the emerging concept of co-generation of hydrogen and power from

fossil fuels, particularly coal. Energy-efficient co-generation of H2, like current

reforming processes, will be dependent on efficient separation of hydrogen from

synthetic gas (syngas) [2-3].

Membrane separation is the most energy-efficient separation technology currently

available because it is a single-pass process, with no need for sorbent regeneration or

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desorption by temperature/pressure variation. Hydrogen-selective membranes that are

sulfur- and steam-resistant, and temperature stable (>400 °C), would be ideal for H2

production applications. Zeolite membranes have been of particular interest because they

have a unique molecular sieving mechanism and relatively high tolerance to sulfur and

steam at high temperatures [4-7]. Synthetic molecular sieve membranes for hydrogen

separation have been studied intensively; however, their applications have been limited

by high production costs and technical challenges including cracks or defects in the

membranes, and poor physical and chemical compatibility between thin synthetic

membranes and the obligatory porous supports [8, 9]. In addition, despite active research

and development in hydrogen separation by synthetic molecular membranes [10-12],

very little attention has been paid to the hydrogen gas permeance and selectivity of

natural zeolite-based membranes and their potential applications in the hydrogen

separation industry.

Clinoptilolite is one of the most common naturally occurring natural zeolites, but

the clinoptilolite from Castle Mountain in Australia appears to be a unique deposit. The

material has been compressed by its environment to the point that it has essentially no

macroporosity. With a bulk density approaching 2.7 g/cm3, approximating the value

expected for a single clinoptilolite crystal, this material may be regarded as a solid zeolite

block. In this report, we describe hydrogen separation though both raw and surface-

modified, clinoptilolite-rich natural zeolite membranes. These rugged and economical

natural zeolite membranes selectively separate hydrogen from CO2, CH4, C2H4, and C2H6,

and the surface-modified membrane is thermally and chemically stable after multiple

heating-cooling cycles, indicating that it may be suitable for industrial hydrogen

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separations. Compressed Castle Mountain clinoptilolite may also serve as a model for

the development of synthetic membranes that combine improved separation properties

with increased mechanical strength.

2. EXPERIMENTAL

2.1 Materials

The natural zeolite rocks purchased from Castle Mountain Zeolites (Quirindi, NSW,

Australia) are clinoptilolite-rich minerals composed of clinoptilolite (~85 wt.%) and

mordenite (~15 wt.%) with trace amounts of quartz. The nominal mineralogical

composition of Castle Mountain zeolite is listed in Table 1 and the Si/Al molar ratio

calculated from the composition data is 5.03.

2.2 Membrane preparation

The rocks were sectioned by a diamond saw into thin discs approximately 1.25 cm in

diameter and 1.0 - 1.5 mm thick. The discs were polished with a diamond polishing lap

(180mesh, Fac-Ette Manufacturing Inc.) followed by washing in an ultrasonic bath for 30

min. Clean discs were dried in an oven at 120 °C for at least 2 h.

2.3 Surface modification

A dried, clean membrane was supported on the top of a glass vial, which was placed

in the center of a Teflon-lined autoclave. A few drops of deionized water were added into

the support vial and some water was also added to the area surrounding the vial; the

membrane did not directly contact the water. Two drops of surface modification solution

were added to the top surface of the membrane and the autoclave was immediately

sealed. The surface modification solution was a thoroughly mixed solution of sodium

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silicalite, sodium hydroxide and deionized water with a weight ratio of 5:1:14. Surface

modification took place in an oven at 150 °C for three days. The modified membrane was

then washed with deionized water, gently polished with a silicon carbide sand paper

(400mesh) and washed again, and finally dried at 120 °C for at least 2 hours.

2.4 X-ray diffraction (XRD) and scanning electron microscopy (SEM) of raw and

surface-modified membranes

Data for phase identification of raw and surface-modified membranes was collected

by XRD using a Rigaku Geigerflex Model 2173 diffractometer with a Co tube and a

graphite monochromator. The surface morphology of the membranes was examined by

SEM using a JEOL 6301F field emission scanning electron microscope supplemented

with energy dispersive x-ray spectroscopy (EDX).

2.5 Gas permeation measurement

A schematic diagram of the experimental setup for gas permeation measurements is

shown in Figure 1. Briefly, the membrane was sealed on the top of an inner ceramic tube

(OD=1/2”) with a glass sealant (SG-683 K; Heraeus GmbH, Germany). The feed side

was a quartz tube. The permeation side consisted of two ceramic tubes arranged in a tube-

shell structure with a 1/4" ceramic tube for sweeping gas in and a 1/2” ceramic tube shell

for sweeping gas-carried permeate out. During gas permeation measurements, the flow

rate of the feed side was maintained at 100 mL/min (STP), while the flow rate of the

sweeping gas (Ar) was maintained at 200 mL/min (STP). The entire permeation system

was placed into a tube furnace and the temperature was controlled by a multipoint

programmed temperature controller.

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Gas permeation of each pure gas (supplied by Praxair Canada, Inc.) was measured at

temperatures ranging from 25 °C to 500 °C with a partial pressure difference across the

membrane of approximately 100 kPa. An on-line gas chromatograph (GC; Shimadzu GC-

14B) equipped with a HayeSep Q packed column and a thermal conductivity detector

was used to analyze the outlet gas composition from both the feed and permeation sides.

3. RESULTS AND DISCUSSION

3.1 Characterization of raw and surface-modified natural zeolite membranes

XRD analysis of raw and surface-modified natural zeolite membranes confirmed

that the major phases of the natural zeolite membrane material are clinoptilolite and

mordenite, with minor quartz impurities [13]. Although some authors have reported that

hydrothermal treatment of clinoptilolite-rich natural zeolites with a concentrated solution

of either sodium hydroxide or alkali sodium silicate results in structural changes [14-18],

the XRD patterns of these clinoptilolite-rich raw and surface-modified samples were very

similar (Figure 2). Some changes were observed in the 2θ range of 15 to 40° following

surface modification: the peaks observed at 22.08° and 27.59° in the raw sample

decreased in height, while the intensity of the peaks located at 2 =15.83° and 2=32.32°

increased.

The overall Si/Al ratio of the surface of the modified membrane was determined

to be 7.49 by EDX, much lower than that of the raw membrane (9.17), which may be a

result of the dissolution of Si impurities in the strong caustic modification solution during

the hydrothermal modification process. In addition, the Si/Al ratios for both the raw and

surface-modified samples, based on overall EDX surface analysis, were higher than the

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calculated value of 5.03. EDX point analysis confirmed that most of the membrane

surface after modification consisted of clinoptilolite, which has a reported Si/Al ratio of

4.25 to 5.25 [13]. EDX point examination of the surfaces also revealed that the surface

composition of the membranes varies from point to point, especially for the raw

membrane, a result which is consistent with the composite nature of natural zeolite

minerals. Overall, the surface morphology of the modified membranes appears to include

larger crystals than the surface of the raw membranes (Figure 3a and 3b).

3.2 Gas separation performance

The following equations were used to calculate the permeance and selectivity:

Pi=

N i

Δpi ,

where Pi is the permeance (mol/m2·s·Pa), Ni is molar flux (mol/s m2) of component i, pi

is the partial pressure difference (Pa) of component i across the membrane; and

Sij=

Pi

P j ,

Sij is the ideal selectivity of i over j.

The permeance results for seven raw membrane samples for H2 and four raw

membrane samples for CO2 are shown in Figure 4. For all the permeance tests at

temperatures above 100 C the standard deviation was less than 20%, presumably due to

very low concentration of gas components in the samples. As expected, both higher and

lower than average hydrogen permeances and unstable permeances of CO2 (Figure 4)

were observed in some as-mined samples. This is possibly due to the presence of

impurities, or the potential phase transformation of unstable zeolite crystals at

temperatures exceeding 400 °C [14]. The gas permeance and H2 ideal selectivity of raw

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natural zeolite membranes are shown in Figure 5. An average hydrogen permeance

approaching 2.510-7 mol m-2 s-1 Pa-1, and selectivity for hydrogen over CO2 and C1/C2

hydrocarbons close to or slightly higher than that predicted by Knudsen selectivity (

Sij=√ M j

M i , Mi, Mj – molecular weights of i and j species) were observed at all

temperatures tested using raw sections of this natural clinoptilolite material. This is

consistent with the dense and compacted natural characteristics of this zeolite mineral.

The permeance and the hydrogen selectivities of the raw membranes used in this study

indicate that these materials contain essentially no macroporous defects. The absence of

macroporosity suggests that this compacted natural clinoptilolite may be a good

candidate membrane material for hydrogen separations.

The gas permeation and ideal selectivity characteristics of surface-modified natural

zeolite membranes are shown in Figure 6. At all temperatures tested, the surface modified

membranes demonstrated significantly improved selectivity for H2 over CO2 and C1/C2

hydrocarbons, while their hydrogen permeance was essentially unchanged. The hydrogen

permeance of the modified natural zeolite membranes increased with operating

temperature, doubling from to ~2.5×10-7 mol.m-2.s-1Pa-1 at 25 °C to ~5×10-7 mol.m-2.s-1Pa-1

at 500 °C. It is noteworthy that the approximately 1.2 mm thick membranes used for

these experiments had permeance values that are comparable to the permeation results

reported in the literature for thin supported synthetic membranes (thickness of 3~60 µm)

[19-23].

The experimental results show that the permeance of CH4 (kinetic diameter: 0.38 nm)

was higher than that of the smaller CO2 (kinetic diameter: 0.33 nm). Also, the

permeances of all gases tested increased with the operating temperature (Figures 5a and

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6a). Because of the increasing trend with temperature observed for the permeation results

(opposite to the decreasing trend with temperature associated with Knudsen diffusion [24,

25]), the zeolite flux contribution is probably the prevailing transport mechanism at these

experimental conditions. Thus, at room temperature, a higher adsorption strength may

well lead to a lower diffusivity value for CO2. For instance, CO2 was predicted to diffuse

slower than CH4 in MFI zeolites at 300K using molecular dynamics [26]. At higher

temperatures, with a weak adsorption affinity, molecules are expected to permeate based

on an activated gaseous diffusion regime [27 - 29]. The strong temperature dependence of

gas permeance for both raw and modified membranes indicates that diffusion through the

natural zeolite membranes is an activated diffusion process [27, 28].

The selectivity for hydrogen over CO2 and C1/C2 hydrocarbons was significantly

higher than that predicted by Knudsen diffusion for the modified membranes. For the

surface-modified membranes, the ideal selectivity for hydrogen over CO2 was 57 at 25 °C

and 13 at 500 °C. The ideal selectivities for hydrogen over CH4, C2H4 and C2H6 were 22,

98 and 78 at 25 °C and 6, 26 and 12 at 500 °C, respectively (Figure 6b). The

corresponding Knudsen separation factors are 4.69 (H2/CO2), 2.83 (H2/CH4),

3.74(H2/C2H4), and 3.87(H2/C2H6). High H2 selectivity indicates that gas separation

through the modified natural zeolite membrane is dominated by molecular sieving and

adsorption/diffusion mechanisms. The higher selectivity observed for hydrogen over

C2H4, even at temperatures as high as 500°C, may be attributed to the strong adsorption

behavior of C2H4 on the zeolite surface. A study to elucidate the correlation between the

adsorption/diffusion behaviour of the gases and their membrane separation characteristics

is in progress.

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3.3 Thermal and Chemical Stability

The hydrogen permeance of raw and upgraded (surface-modified) membranes was

examined both before and after three heating/cooling cycles (Figure 7). The H2

permeance of the raw membrane was significantly reduced after repeated heating-

cooling-permeation test cycles (Figure 7a). In contrast, the surface-modified membrane

showed higher thermal and chemical stability, as evidenced by reproducible hydrogen

permeance after 3 cycles of heating-cooling combined with hydrocarbon permeation tests

at 500 °C (Figure 7b). Consistent with the results of XRD, SEM and EDX analysis, the

improved thermal stability of the surface-treated membrane suggests that the

hydrothermal modification process may remove or transform less stable impurities or

phases within the natural zeolite. Pretreating natural zeolite clinoptilolite with sodium

hydroxide also has been reported to improve the NH4 adsorption capacity of clinoptilolite

through the hydrothermal transformation of clinoptilolite[16,17].

Based on the results of this work, we propose that the improved H2 selectivity of

natural zeolite membranes observed following hydrothermal modification may be a result

of several simultaneous processes, including in-situ transformation of the crystalline

structure within the zeolite, removal of Si impurities from the surface and healing of the

nanoscale defects inherent to the mined mineral. Further characterization and

quantification of the properties of both the raw and surface-modified natural zeolite

membranes will be required to provide a conclusive explanation for the properties of

surface-modified natural zeolite membranes.

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4. CONCLUSIONS

Compacted, metamorphosed zeolite clinoptilolite films can act as natural

molecular sieve membranes. High hydrogen permeance (as high as 610-7 mol m-2 s-1 Pa-

1) and ideal selectivity for hydrogen over CO2, CH4, C2H4, and C2H6 up to two times

better than predicted by Knudsen diffusion can be achieved using raw sections of this

material. Further improvements to stability and selectivity can be achieved by chemical

surface modification. Simple hydrothermal surface modification of slices of raw,

clinoptilolite-rich natural zeolite results in 1.2 mm-thick membranes with hydrogen

permeance as high as 5.210-7 mol m-2 s-1 Pa-1 at 500 °C, and selectivities of 57 (H2/CO2),

22 (H2/CH4), 98 (H2/C2H4) and 78 (H2/C2H6) at 25 C; and 13 (H2/CO2), 6 (H2/CH4), 26

(H2/C2H4) and 12 (H2/C2H6) at 500 C. The modified membranes also exhibit high

thermal and chemical stability after several heating-cooling temperature cycles. This

study demonstrates that zeolite membranes made directly from this unique, compacted

natural zeolite may have potential for selective separation of hydrogen from other

industrial gases in high-temperature applications. In addition, this rugged natural zeolite

may provide a model for the development of rugged new synthetic zeolite membranes

with superior separation properties.

ACKNOWLEDGEMENTS

The authors thank Dr. Amy Dambrowitz for assistance with manuscript

development. The authors gratefully acknowledge the research funding provided for this

program by Nova Chemicals Corporation and the Alberta Energy Research Institute.

This work was also supported by the Alberta Ingenuity Fund through the Scholar in

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Separation Technology for Oilsands Extraction and Upgrading (S.M.K.), and the Natural

Sciences and Engineering Research Council Industrial Research Chair in Molecular Sieve

Separations Technology (S.M.K.).

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Selectivity of Clinoptilolite for Metal Ions, Water Res. 22 (1988) 537-542.

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Table 1. Chemical composition of Castle Mountain zeolite*

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Mineral content wt.%

SiO2 71.81

Al2O3 12.10

Fe2O3 1.14

Na2O 2.33

K2O 0.90

CaO 2.60

MgO 0.65

TiO2 0.22

MnO 0.03

P2O5 <0.01

SrO 0.22

Loss on Ignition 7.77

* Information provided by the supplier

FIGURE CAPTIONS

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Figure 1. Schematic of the set-up for gas permeation measurements.

Figure 2. XRD Patterns of raw and surface-treated zeolite membranes.

Figure 3. SEM images of the surfaces of raw (a) and surface-modified (b) natural zeolite membranes.

Figure 4. Permeances of H2 and CO2 through membranes from different natural zeolite rocks.

Figure 5. Gas permeation (a) and selectivity for H2 over CO2, CH4, C2H4, and C2H6 (b) on a raw natural zeolite membrane.

Figure 6. Gas permeation (a) and selectivity for H2 over CO2, CH4, C2H4, and C2H6 (b) on a surface-modified natural zeolite membrane.

Figure 7. The stability of H2 permeance after 3 heating/cooling cycles for raw (a) and upgraded (b) natural zeolite membranes.

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Figure 1.

18

010 20 30 40 50

2-Theta

Cou

nts

RawUpgraded

Figure 2.

19

a.

b.

Figure 3.

20

1.0E-08

1.0E-07

1.0E-06

0 100 200 300 400 500 600

Temperatures (oC)

Perm

eanc

e (m

ol/m

2 .s.P

a)

H2

CO2

Figure 4.

21

a.

0.00E+00

5.00E-08

1.00E-07

1.50E-07

2.00E-07

2.50E-07

3.00E-07

3.50E-07

0 100 200 300 400 500 600

Temperature (oC)

Per

mea

nce

(mol

/(m2 •s

•Pa)

)

H2CO2CH4C2H4C2H6

b.

0

3

6

9

12

15

0 100 200 300 400 500 600

Temperature (oC)

Sel

ectiv

ity

H2/CO2H2/CH4H2/C2H4H2/C2H6

Figure 5. a.

22

0.00E+00

1.00E-07

2.00E-07

3.00E-07

4.00E-07

5.00E-07

6.00E-07

0 100 200 300 400 500 600

Temperature (oC)

Per

mea

nce

(mol

/(m2 •s

•Pa)

)

H2CO2CH4C2H4C2H6

b.

0

20

40

60

80

100

120

0 100 200 300 400 500 600

Temperature (oC)

Sel

ectiv

ity

H2/CO2H2/CH4H2/C2H4H2/C2H6

Figure 6. a.

23

0.0E+00

2.0E-07

4.0E-07

6.0E-07

8.0E-07

1.0E-06

0 100 200 300 400 500 600

Temperature (oC)

Per

mea

nce

(mol

/(m2•

s•P

a))

Fresh Sample

After 3 Temperature Cycles

b.

0.0E+00

2.0E-07

4.0E-07

6.0E-07

8.0E-07

0 100 200 300 400 500 600

Temperature (oC)

Per

mea

nce

(mol

/(m2•

s•P

a))

Fresh Sample

After 3 Temperature Cycles

Figure 7.

24